This project aims to elucidate regulatory mechanisms of gonadotropin secretion with particular reference to steroid feedback and to changes in pituitary sensitivity to luteinizing hormone releasing factor (LHRF). Radioimmunoassay is being used to study, in the rat, changes in circulating gonadotropin levels in response to such stimuli as (1) administration of synthetic LHRF and (2) copulation. These changes will be studied following feedback suppression by estrogen, testosterone and progesterone and following hypothalamic lesions. The "positive feedback" effect of estrogen on LH secretion will also be studied with a view to elucidating possible actions of the ovarian steroid at the hypothalamic and/or pituitary levels. Localization of the various steroid feedback effects will be investigated by comparing the effects of systemic administration and local intracranial implantation of the crystalline hormones. The effects of stress on gonadotropin and gonadal steroid levels will also be studied.